Magnetic resonance in spherical Co-Ni and Fe-Co-Ni particles
Magnetic resonance in spherical Co-Ni and Fe-Co-Ni particles
复制标题
DOI：
10.1103/physrevb.62.532
复制
发表时间：
2000-07-01
影响因子：
3.7
通讯作者：
Acher, O
中科院分区：
文献类型：
--
作者：
Mercier, D;Lévy, JCS;Acher, O
研究了球形和单分散的Co-Ni和Fe-Co-Ni微粒复合材料在无外场作用下的磁共振行为。颗粒的大小范围在一个数量级上变化，从25到250 nm。在所研究的频率范围[0.1-18 GHz]内，通常观察到几个非均匀共振模的共振带。研究发现，共振频率依赖于磁性粒子的大小，对于最低频率模式，只有微弱的依赖关系，而对于以下模式，这种依赖关系更为明显。提出了一种基于离散处理的独立球形或柱状小颗粒中共振效应的理论模型。它允许计算共振频率、自旋波分布和自旋波强度。给出了颗粒几何形状、表面钉扎、晶体各向异性和交换参数值的各自影响。根据该模型，发现共振频率的尺寸依赖关系主要与表面钉扎和交换参数值有关。在所有情况下，最低频率模式的尺寸依赖性都弱于以下模式，并且对于大颗粒，第一模式的频率与晶体各向异性直接相关。粒子形状对共振频率的影响很小，而第一模的自旋波分布依赖于粒子的几何形状。这个模型使我们能够描述实验光谱的一般形状，并从实验数据中推断出磁晶各向异性常数(KI)，这些结果与体积值很好地吻合。此外，它还表明，最低频率模式的弱尺寸依赖性是由于粒子表面的弱钉扎所致。然而，颗粒大小对较高自旋波模式的影响要求我们考虑颗粒之间的磁相互作用。
Magnetic resonance is studied in absence of external field on composite materials made up with spherical and monodisperse fine Co-Ni and Fe-Co-Ni particles. The particle size range varies over one order of magnitude from 25 to 250 nm. In the frequency range studied [0.1-18 GHz] several resonance bands are generally observed attributed to nonuniform resonance modes. The resonance frequencies are found to depend on the magnetic particle size, only weakly for the lowest frequency mode, in a more pronounced manner for the following modes. A theoretical model based on a discrete treatment of the resonant effect in independent small spherical or cylindrical grains is proposed. It allows to compute the resonance frequencies, the spin-wave profiles, and the spin-wave intensities. The respective influences of particle geometry, surface pinning, crystalline anisotropy, and exchange parameter values are presented. According to this model it is found that the size dependence of the resonance frequencies is mainly related to the surface pinning and to the exchange parameter value. In all cases the size dependence of the lowest frequency mode is found weaker than that of the following modes and for large particles the frequency of the first mode is directly related to the crystalline anisotropy. The particle shape effect on resonance frequencies is weak whereas the spin-wave profiles of the first modes are found to depend on the particle geometry. This model enables us to describe the general shape of the experimental spectra and to infer the magnetocrystalline anisotropy constants (KI) from the experimental data, which are found in good agreement with bulk values. Moreover it shows that the weak size dependence of the lowest frequency mode is due to a weak pinning at the particle surface. Nevertheless, the effect of particle size on higher spin-wave modes requires us to account for the magnetic interactions between grains.